Optical heart rate sensors are well known to monitor or detect vital signs like a heart rate of a user. Such a heart rate sensor can be based on a photoplethysmograph (PPG) sensor and can be used to acquire a volumetric organ measurement. By means of pulse oximeters, changes in light absorption of a human skin is detected and based on these measurements a heart rate or other vital signs of a user can be determined. The PPG sensors comprise a light source like a light emitting diode (LED) which is emitting light into the skin of a user. The emitted light is scattered in the skin and is at least partially absorbed by the blood. Part of the light exits the skin and can be captured by a photodiode. The amount of light that is captured by the photo diode can be an indication of the blood volume inside the skin of a user. A PPG sensor can monitor the perfusion of blood in the dermis and subcutaneous tissue of the skin through an absorption measurement at a specific wave length. If the blood volume is changed due to the pulsating heart, the scattered light coming back from the skin of the user is also changing. Therefore, by monitoring the detected light signal by means of the photodiode, a pulse of a user in his skin and thus the heart rate can be determined.
In other words, information regarding cardiovascular status like blood oxygen saturation, heart and respiratory rates can be unobtrusively acquired by a PPG sensor. However, PPG sensors can be susceptible to motion-induced signal distortions.
EP2116183 B1 discloses an opto-electrical ear located cardio vascular monitoring device which comprises two infrared light emitting devices emitting at two different wavelengths, wherein this light is detected by two pairs of photo diodes. Furthermore, the device comprises a motion sensor for detecting the motion of a user. If the user is at rest or is moving too irregularly, one of the cardio vascular sensors can be switched off. The usage of two different light emitting diodes will, however, increase the power consumption of the sensor.